Speed changing is indispensable. Frequently a prime mover must work at high rotational speeds for optimized efficiency while the load it drives must run at tenths or even hundredths its speed. One way to obtain such large speed reduction ratio is to use a cascade of reducers of smaller ratio but at best efficiency.
However, this cascaded speed reduction suffers poor overall speed-changing efficiency due to its nature of having the entire load passing successively through each and every reducer stage of the cascade. The arrangement is also bulky for the obvious reason that each stage in the cascade must be fully rated to handle one hundred percent of the total power delivered by the prime mover.
One type of “single-stage” reducers currently used widely is the cycloidal drive manufactured by Sumitomo Heavy Industries, Ltd. of Tokyo, Japan. Although relatively compact for speed-changing ratios ranging from tens to more than one hundred, the drive is, essentially, one cycloidal gearing stage followed by an off-axis power extraction stage.
FIG. 1 schematically illustrates the configuration of such a cycloidal speed reducer in cross section. The conventional device in FIG. 1 has a fixed ring gear 11 and a shaped planet element 12, sometimes a shaped disc or sometimes simply a gear. The planet element 12 engages with and moves inside the ring gear 11 epicyclically. The two have an as-small-as-possible difference in their working pitch diameters.
For the off-axis power extraction stage, a disc 13 is fixed to the planet element 12 coaxially on their axis 19 and has a number of holes 17 to allow for engagement by a corresponding number of roller pins 18 planted on the plate 14. The plate 14 is coupled to the output shaft 16 of the drive and is centered on the central axis 10 of the device. This “power extraction” arrangement allows the drive to deliver a speed-reduction ratio of −K/i, wherein K is the pitch diameter of the planet element 12 and i is the difference between the pitch diameters of elements 11 and 12. In a typical example wherein the ring gear 11 has 80 teeth and a gear version of the planet element 12 has 79 (K=80 mm and i=1 mm using module 1 metric gears), the ratio is −80 when mechanical power is transmitted by the device via the input at shaft 15.
FIG. 2 schematically illustrates the off-axis power extraction coupling used for the prior art cycloidal drive of FIG. 1. At any given time, only one of the typically eight or more pin-roller and cycloidal disc hole engagements is transmitting torque fully. For example, with the angular position of the relative offset and with the direction of rotation as shown, only the pair of pin roller 18C and hole 17C is transmitting power fully for the device.
This is obvious as the edge of the hole 17C of the driving disc 13 that is in contact with the pin roller 18C of the driven plate 14 must be behind the roller 18C along the direction of rotation. In this sense pin roller and hole pairs identified by B and D are partially working to transmit power because of the location of their contact points relative to the direction of rotation of the disc 13 and plate 14. In the same sense, the pin-roller and hole pair 18G and 17G is not working at all because the pin roller 18G, the driven, travels behind its contact point with its hole 17G, the driver.
Conventional cycloidal drives rely on a synchronizing engagement between two elements (gears) of different pitch diameter with offset axes. But this is not an optimized mechanism due to low utilization: Of all eight pin/hole pairs shown in FIG. 2, half (four or even five depending on the angular position) of them are not in the position to drive the load. Of the other half, only one can be in a full-effort position to drive the load, the other three are in their partial effort. With limitations such as these, cycloidal drives achieve typically less than 80 percent efficiency under normal load conditions.
Further, to achieve a speed reduction ratio of K, a cycloidal drive requires a fixed ring gear of K+1 teeth. For large ratio, the large ring gear number makes the drive bulky if the rated torque is substantial therefore the teeth must be sufficiently robust—in size. In other words, compactness of the cycloidal drive places a limitation on the torque and power rating of the drive.
Another type of large-ratio reducer widely used in precision and aerospace applications is the harmonic drive manufactured by Harmonic Drive Systems Inc. of Tokyo, Japan. Operating the basic concept known as strain wave gearing, harmonic drive is relatively low in available power rating. The drive also delivers typically less than 60 percent efficiency under normal load because its spline element flexes all the time as the drive operates to transmit mechanical power.
In addition to large-ratio speed reducers there are also the need to increase a slow input speed to an output up to tens or hundreds of times faster.